This invention relates generally to devices for mixing gases with fluids and for simultaneously agitating those fluids, and particularly to a device for aerating waste water in sewage treatment reservoirs and settling ponds.
The underlying principles involved in waste water aeration have been adequately explained in U.S. Pat. Nos. 4,240,990; 4,280,911; and 4,308,221. When aerating waste water to neutralize pollutants and promote the growth of those aerobic bacteria necessary for certain purification treatment methods or composting operations, there are two general goals. The first is to introduce the maximum amount of oxygen into the liquid that can be achieved. It is thus favorable to introduce the oxygen in the form of discrete bubbles of minimal size which will increase the diffusion rate and oxygen transfer efficiency for a given aerator. The second goal is to agitate the liquid, particularly at the point of oxygen injection. Complete agitation ensures that maximum gas diffusion and oxygen transfer will be achieved, since agitation both increases the number of discrete gas bubbles present at the point of injection and the flow rate of liquid through the area surrounding that point.
Devices which aerate and agitate stagnant waste water in water treatment ponds or reservoirs are conventionally mounted on a shoreline embankment, dock, or within a treatment facility building. The devices are commonly comprised of a motor drive unit, or power head which is situated above the water line, and a hollow drive or impeller shaft which also serves as a gas conduit which extends angularly downward below the surface of the water.
U.S. Pat. Nos. 4,240,990; 4,280,911; and 4,308,221 disclose an apparatus in which a shaft driven propeller coupled to a motor is mounted within a vortex shield at the distal end of the aerator. The drive shaft itself contains a hollow interior tube which terminates in an open end near just below the propeller. As the propeller is rotated at high speeds, the water is agitated and expelled outwardly through the vortex shield. The flowing water produces a slight vacuum or venturi effect which draws air into the hollow interior tube through inlet slots located near the drive motor, with that air being expelled into the agitated water from the distal end of the hollow interior tube.
These patents also disclose the use of a plurality of plates or vanes situated adjacent to the open distal end of the hollow interior tube opposite the propeller blades which function to break the bubbles of gas being expelled from the hollow interior tube into smaller, finely divided bubbles. These more discrete bubbles serve to lower the dependence on "hang time" for dissolving the oxygen into the water by increasing the total aggregate surface area of the multitude of singular bubbles, thereby increasing the oxygen transport interface.
U.S. Pat. Nos. 3,782,702 and 4,448,685 each disclose the use of a screw-type impeller blade in combination with an aerator device. The '685 patent employs a cylindrical helical screw impeller, while the '702 patent incorporates a pair of oppositely oriented continuous spiral screw blades. Similar screw blade configurations are shown in U.S. Pat. Nos. 4,200,597 and 4,230,648.
U.S. Pat. No. 3,975,469 discloses an aerator device for revolving liquids and supplying a gas thereto having at least one continuous spiral screw blade which forms an acute angle to the drive shaft, and further incorporates a foam knife which rotates on the drive shaft at the surface of the water. This patent further describes other commercial uses for aerator devices such as precipitating divalent iron from water by oxidation to three-valent iron, or removing carbonic acid from steam boiler water by aeration.
U.S. Pat. No. 3,778,233 discloses an apparatus for liquid composting of animal wastes which is designed to be used in the reservoir situated below the flooring of a livestock confinement building. The aerator disclosed in the '233 patent is similar in design and operation to that of the above mentioned '990 patent, however the '223 patent provides for an elongated stabilizing cylinder surrounding the drive shaft.
As one alternative, U.S. Pat. No. 3,606,273 shows an aerator which may be floated on the surface of the pond, with the impeller shaft extending vertically downward below the water. The '273 patent discloses an improved deflection bearing, deflector hub, and propeller blades mounted to the shaft and situated within the housing, with the propeller blades and lower portion of the housing remaining below the surface of the water such that the entire housing acts as the conduit through which the air is injected.
Other methods exist for mixing gases with a liquid being stirred, agitated, or transferred. Devices for performing these tasks on a smaller scale are commonly found in chemical or biological laboratory settings. The simplest method is to place the liquid in a vessel with a stirring apparatus, and introduce the desired gas into that vessel above the liquid to form a gas-liquid interface. Another similar method is to bubble the gas through the liquid, or pass the liquid through a gas filter or over a gas transfer surface. One can thus rely upon the natural rate of gas transfer, which depends upon the chemical properties of the gas and liquid, the surface area of the transfer, the diffusion rate and the agitation of the liquid, the effect of the temperature of the liquid on gas solubility, and so forth. One can also stir the liquid to produce such a high angular velocity that a downward syphon or jet is formed on the surface of the liquid, which in turn draws gas from above the gas-fluid interface. Such methods are enhanced by cooling the liquid and increasing the pressure of the volume of gas above the gas-fluid interface by several atmospheres.
One apparatus representative of these methods of mixing gases and liquids in a laboratory or chemical engineering setting is shown in U.S. Pat. No. 4,267,052. The '052 patent discloses a mixing vessel having a hollow, motor driven stirring rotor which draws gas and liquid into its center by a venturi effect, and injects the liquid and gas mixture through the bottom of the rotor. Gas is also transferred to the liquid at the gas-liquid interface.
The aerator devices described above do possess several common deficiencies or limitations. The devices rely solely upon a venturi principle to draw air into a hollow tube or conduit and transport that air to a point below the surface of the water. While the air may either be mixed with the water within the conduit or near the point of injection or agitation at the distal end of the conduit, the air is drawn into the conduit from a source which remains at a relatively constant atmospheric pressure. Due to the vacuum created by the moving water or air responsible for the venturi effect, the air being injected into the water at the distal end of the aerator is generally at a pressure much less than that atmospheric pressure.
The devices each rely upon some type of propeller or impeller for agitating the water. The angle, shape, number, size, and rotational relationship of the blades on those propellers are the factors primarily responsible for determining the degree of agitation or turbulence imparted to the water, and the rate at which a uniform volume of water may be passed through the zone of air injection and mixing. While some aerators are shown having baffles or plates against which the aerated water may be directed to disrupt the bubbles into finer, more discrete bubbles, these baffles or plates create a static pressure adjacent to the zone of air injection which interferes with the maximum flow rate of the water along its longitudinal path through the zone of air injection.